Also, if you already have great glass, there is little reason to get more (unless you need it for a certain reason). E.g., if you have a 70-200 f/4, don't rush out to get the f/2.8 just because, unless you need it. The benefits of getting a new lens is really most seen when going from kit lens (low-end) to the higher end lenses; if you already have good glass, chances are good that you'll see more difference from the body than the glass.

Most kit lenses are 18-55mm, f/3.5-5.6 -- not great. However, some cameras come with 18-105mm f/3.5-5.6 lenses. It's still relatively slow, but it's much better than the kit lenses that come with lower end cameras.

But I'm looking at this from a raw specs prospective. Is there some reason (E.g. glass quality, features, etc) that kit lenses are of poor quality?

EDIT: There seems to be some confusion on why I asked this question -- I'm mainly asking because there seems to be this stigma that kit lenses are worthless and that before using any reasonably expensive body that someone should put a "better" lens on first. I'm wondering how close that is to reality.

9 Answers
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I'm reading your question is basically "what do they do in a more expensive lens to make it better?"

There are a number of things. Quite a bit is simple mechanics: more expensive lenses get better quality assurance, so you have a lot better assurance that the individual lens you get actually performs as well as the design was intended to. Second, is pretty similar: in a more expensive lens, they can afford to use better materials -- in a typical kit lens, most of the mechanical parts are typically made from molded plastics; in a more expensive lens, many of those parts will be metal -- mostly brass or stainless steel.

These are particularly important with zoom lenses (which includes essentially all current kit lenses). A zoom lens has quite a few moving parts, and the mechanical tolerances to get maximum performance from a lens (especially a zoom) are quite tight -- quite literally on the order of wavelengths of light in some cases. As such, better build can/does translate to better optical quality.

Third, is the optical design itself. In a more expensive design it's more reasonable to use things like low-dispersion elements and/or aspheric elements. LD elements are used primarily to reduce chromatic aberration (primarily of interest in telephoto lenses). Aspheric elements are used primarily to reduce spherical aberration (primarily interesting in relatively wide-angle lenses). Most kit lenses cover a range from at least fairly wide-angle to short telephoto, so most of the designs could benefit from using both aspheric elements and low-dispersion elements -- but given the expense, neither is nearly as common in kit lenses as in more expensive designs.

Finally, at least for Canon and Nikon (most other brands build these systems into the body instead of the lens), the quality of VR/IS system varies widely among different grades of lenses. While their kit lenses mostly do include such systems, most tests confirm that they provide substantially less benefit than the versions used in their more expensive designs.

Answering the question your asked in comments: no, not all kit lenses are that horrible. Sony probably has the widest range in this regard: they used to sell and 18-70mm that really was as awful as people like to say -- quite possibly the single worst lens you could get from any manufacturer. Then, about a year ago (I don't remember exactly) they dropped it and replaced it with an 18-55mm that's a lot better. As @John Cavan pointed out, the Pentax is also quite a decent lens. Depending on the focal length and whether you care more about the center or corners, you could make a pretty decent argument for any of the Sony, Pentax or Nikkor being the best kit lens, and in any case all three are really quite decent -- at least when they're new; keep in mind the discussion of mechanical quality above, and keep in mind that it means kit lenses tend to wear out fairly quickly. At the moment, Canon seems to be the only one whose kit lens really fully deserves the horrible reputation (and I haven't kept close track -- they may have upgraded it too).

I feel obliged to add that I think a lot of the bad-mouthing of kit lenses really is somewhat undeserved though. In particular, people mostly start out with a kit lens. A few years later, they look at the pictures they took with the kit lens, and blame the lens rather than themselves for the poor quality.

+1 for the last paragraph -- never thought of it that way.
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Billy ONealJun 16 '11 at 23:34

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@Itai: Oh, I certainly don't mean to say that kit lenses are some great overlooked gem, or anything like that. At the same time, some great photography has been done with lenses that were almost certainly technically inferior even to them.
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Jerry CoffinJun 17 '11 at 15:14

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last paragraph is correct, but only in part. While most people buying a kit will not get the most out of that lens initially, there comes a time when they become limited (if they become more serious photographers) by that lens and run up against its limitations, and that time may come quickly. I was lucky in that regard when I got my kit 30 odd years ago as it included a quite decent 50mm f/2 which served me well for years.
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jwentingJun 20 '11 at 5:36

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about limitations, and in defense of your last paragraph: searching on flickr by interesting photos using your lens as query gives you a pretty good idea of what can be done with it. I usually lose any kind of feeling of "limitation by lens" i may have :)
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JoséNunoFerreiraJun 20 '11 at 15:21

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I have to agree with jwenting. I started with the 18-55mm kit lens, and ran into its limitations within a few months of starting photography. That lens IS quite soft, particularly at the extremes, but it exhibits quite a bit of CA at all focal lengths. Contrast is low, and the lens mount is extremely cheap (my first one broke within a year). There are indeed limitations to the lens, and while not everyone will encounter them or care about them, most serious photographers will need to expand sooner or later.
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jrista♦Jun 20 '11 at 19:01

It's pretty harsh to lump all kit lenses into that same bucket. There's no question that kit lenses sold with entry-level cameras tend to be entry-level lenses, but isn't that to be expected?

If you take an entry-level Canon or Nikon kit, for instance, the kit lens is designed to get a relatively new photographer up and running quickly, and to do so at a competitive price point. If Canon tried to bundle "L" glass with their Rebels, I'd bet they wouldn't sell too many kits, and they'd chase a whole lot of new photographers to Nikon, Pentax, Sony, and the rest.

When people come to this site wanting to know what lens(es) they should buy for their cameras, we invariably tell them to be sure they understand how they intend to use those lenses, because there just aren't any really good one-size-fits-all lenses, and there sure aren't any good, versatile lenses at an entry-level price point. The whole idea of a kit lens is to help new users get to the point where they understand what they're really looking for in that next lens.

When we refer to an entry-level lens as being "poor", then, not only is it "poor" only in the context of specific scenarios where other lenses are better, they're (more importantly) poor only when compared to lenses that cost twice as much, ten times as much, or even more.

Finally, while these lenses are usually lacking in optical speed, build quality, and bells & whistles, many of them are actually pretty passable optically. The 18-55 kit lens for Canon's Rebels, for instance, is a really cheap-feeling lens, but DPReview thought it was right in line for its price point. If you stop and think about it, though, this shouldn't be surprising -- these camera manufacturers want you to be successful with your new entry-level camera, even as you're envisioning how much more successful you'd be if only you had a better lens, bigger flash, maybe a battery grip, and so on, so it's in their interest to give you a competent kit lens.

No, not all kit lenses are poor. For example, the DP Review review on the Pentax 18-55mm kit (version 1) is actually quite good. Another review on version 2 shows improvement on the first version as well. In any event, it's a pretty decent lens and at a knock-out price, but it's not a superstar lens either, you just get more than you paid for I think. :)

So, generally, kit lenses are pretty weak as far as it goes, but there are some that step out of that morass and into a bit better a grade.

+1 for an example of an exception, there's always an exception!
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ChrisFletcherJun 16 '11 at 22:05

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I am surprised the bar of mediocrity is so low for kit lenses that the one you mention would be considered quite good. Yes, it is sharper than other kit lenses but if you look at the tests linked above you will notice terrible vignetting pretty clearly.
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ItaiJun 17 '11 at 1:44

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@Itai - Its all relative, but there are lenses that carry much bigger prices and suck far harder. Like I said, it's no superstar, but for the price, it is miles beyond the competition.
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John Cavan♦Jun 17 '11 at 2:40

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@Itai - Value for money is a reasonable measurement and can be considered a factor for a lot of consumers.
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John Cavan♦Jun 17 '11 at 13:07

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Given that the Pentax 18-55mm WR has the same optics as the 18-55mm II and is weather sealed, this would put it well ahead of the other kit lenses assuming that the image quality wasn't significantly different. What's the cheapest weather sealed lens you can buy on the other systems?
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CadentOrangeJun 20 '11 at 6:41

It would entirely depend on the level of the camera kit you are buying. In most cases, lower-end entry-level cameras, such as the Rebel series from Canon or the Nikon D3100, usually do have rather cheap 18-55mm lenses bundled with them. The optics are usually not top notch, build quality is usually lower, however you are getting such a lens for a real steal (the lens itself is usually less than $100 of the total cost of the kit, which may range from $500 to $1000 total.)

On the flip side, if you go out and buy a full Canon 5D Mark II kit, the standard bundled "kit lens" is the 24-105mm f/4 L IS USM lens. The 24-105 is a superb lens, one of Canon's top sellers, offering all the bells and whistles of their luxury line of lenses such as solid build quality, top shelf glass, and some of the best IS and AF Canon lenses have to offer.

Mid-grade cameras usually come bundled with mid-grade lenses that are a step up from your entry-level kits, and a step or two down from the professional-grade kits. Mid-grade lenses usually offer better build quality and additional features (such as ultrasonic-type AF rather than simple servo/gear driven AF), etc. Ultimately...you get what you pay for, and kits come in a variety of bundles.

@jwenting: As far as I know, you can get any canon body except the 1D series as part of a "kit". The pro 5D II comes with the 24-105mm L, the pro 7D comes with either the 28-135mm gold band or the 18-135mm silver band, and the xxD & Rebels usually have a variety of different kits, the most common of which is the 18-55mm followed by the 28-135mm.
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jrista♦Jun 20 '11 at 6:54

surprises me as a Nikon user. Nikon clearly markets their pro equipment at people who're unlikely to buy kits as they either have specific needs not met by a kit or already have equipment and are buying to supplement rather than start from scratch so have no use for a kit.
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jwentingJun 20 '11 at 7:28

@jwenting: You can, obviously, buy any Canon camera as body-only, if you are already invested in lenses. Its a marketing thing...you get a kit for cheaper overall.
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jrista♦Jun 20 '11 at 18:01

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Well, if you think about the place the 5D II holds, its not quite so surprising. A significant number of photographers who might be considered amateur or semi-professional or just a hobbyist of some kind (i.e. astrophotography augmenting their passion for astronomy) buy the 5D II as their first camera. The "true" professional lines from Canon, the 1D series, are still body-only.
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jrista♦Jun 21 '11 at 15:42

Assuming you mean a kit lens as something like an 18-55mm (Canon do sell professional grade lenses as 'kit lenses' if you buy an expensive enough camera too, but ignoring those) then I think it's fair to say all kit lenses are poor....but it's worth pointing out that 'poor quality' is a comparative term, it's possible to take great photos with lower end equipment still.

The reason they're poor is a combination of the factors you listed, generally kit lenses tend have the following drawbacks (these are what come to mind, there are likely more):

Higher levels of chromatic aberration

Higher levels of barrel and/or pincushion distortion

They tend to be lacking the ability make manual focus adjustments while AF is enabled (called Full Time Manual on Canon lenses)

They tend to be made with cheaper less sturdy materials, ie softer plastics

They lack weather resistance (unlike some more expensive lenses)

They can also lack image stabilisation

They tend to be 'slower' as a result of having a smaller aperture

Ofcourse as a result of this they're lighter, cheaper and easier to get hold of.

The reason manufacturers' make 'poor quality' kit lenses is to lower the barrier of entry to prospective buyers, cheaper lenses allow more people to buy into photography in the first place meaning more people will upgrade later.

The EFS 17-85 IS f/4-5.6 kit lens that came with my 30D is proof of point 1 and 7. (Probably more but none that I've really noticed)
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Vian EsterhuizenJun 16 '11 at 21:39

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#7 is pretty much the first reason folks seem to upgrade - you just got a fancy DSLR, but you can't get all those fancy shallow DoF shots you see from the pros (in most situations).
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rfuscaJun 16 '11 at 21:51

It should be kind of obvious. Manufacturers universally charge more for high-end optics because quality has value. Look at the price difference between a body-only and a body+kit-lens and you can easily figure out how much the camera maker values its kit-lenses.

Higher quality lenses cost more because they use better optics and other components. While this is not part of the pure spec, images from cheap lenses are often softer with higher vignetting, stronger distortion, more chromatic aberrations and usually all-of-the-above to some extent.

The most classic kit-lens is basically a disposable lens, it gives neither quality nor versatility but lets you start shooting with your new camera right away.

In terms of specifications this is seen as a short zoom range (18-55mm for APS-C or 14-42mm for 4/3 is typical) and narrow aperture (F3.5-5.6 is extremely common). The aperture is range is problematic not only because you have less play with depth-of-field but also because lenses rarely perform well wide-open. However, when wide-open is F/5.6 and you need to stop down to F/11 you get dangerously close to the diffraction limit which seriously constraints your options.

EDIT For Comment:

It is hard to be more specific then higher quality optics. Just like most manufacturer items, the higher quality ones are produced with higher tolerance, more pure materials to begin with and treated with more sophisticated coatings. This is not limited to just individual elements but how they interact and compensate for each-other's aberrations.

That bad is relative. There is certainly a difference between kit-lenses from various manufacturer and even within samples of the same kit lens. I am in a position to have used various samples of nearly all kit-lenses from all major manufacturer and I can say that their quality is is below what I consider acceptable for any of my professional images. Snapshots of the kids, maybe. I almost bought one from myself while my most important lens was being repaired but I couldn't, instead I bought 4 prime lenses to cover the range loosely which produced much better results.

Some brands ship kit lenses of better quality than the others. It seems to inversely correlate with the market share, the less popular brands tend to ship better kit lenses.

Usually kit lenses do not have as good image quality as more expensive glass, and are not very good for low-light conditions, but some of them are still usable in less demanding situations, also they are usually very lightweight. I think some of the better kit lenses are those by Pentax and Olympus.

How does the image quality suffer (in general, there may be some exceptions):

kit lenses are less sharp, especially in the corners of the frame

stronger chromatic abberations (again, more pronounced in the corners of the frame)

stronger vignetting (up to one stop)

stronger flare (when shooting against the light)

image quality is not consistent across the whole zoom range

Usability-wise, there are the following limitations:

smaller maximum aperture, at 35 mm (≈ 50 mm equiv.) kits can usually go at most f/4.5, to shoot in low light have to use the wide angle zoom position.

slower or less precise autofocus

less effective vibration reduction (if applicable)

Apart from lower optical and build quality, kit lenses may have other limitations to consider:

rotating front element (inconvenient for users of the polarizing filters; Pentax and Olympus kits are good, but Canon and Nikon kits have a rotating front)

lack of or ineffecient lens hood (a very useful item for shooting outdoor; Canon and Nikon kits usually don't have it or it is not a good one, Pentax and Olympus usually have a good lens hood, but at least on Pentax K-x the kit comes without a hood)

yes, I was slightly surprised, that my K-x with kit lens did not come with hood. However, it was relatively easy to get a much cheaper hood - almost a copy of the original Pentax hood from Ebay. :-)
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JuheleJun 20 '11 at 11:47

The discount of kit lens (compared to buying separately) comes from high volume and expected future sales. High volume can only be achieved by picking a lens that almost anybody would want. For lower price cameras, this would have to be the cheapest lens that can cover all the basics - wide, normal, tele. If anything, a bit more tele reach can generate enough interest to justify offering them as (more expensive) kits. People crazy sophisticated enough to choose Pentax might also go with a normal prime. Anything else would

make the kit too expensive for those consumers - they don't expect price to double when including a lens;

and make it less probable that the buyer will upgrade the lens (which is how the camera company is expecting to make money).

People who buy higher end bodies would not be interested in those cheapies, so these come with better lenses in kit. For example, Canon 5d II + 24-105 f/4 IS. If you want f/2.8, you already know where the shop is :)

I would hope most reputable places to get kits (e.g. Amazon) would not have serious fake problems.
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Billy ONealJun 17 '11 at 6:07

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I've never seen or heard of an actual fake lens. Lenses are difficult enough to make that a knock-off good enough to pass would have to be also expensive to make — possibly more expensive than the name-brand one, because the generic versions might not have the same economies of scale as the real version. I'm willing to be convinced, though — do you have an example?
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mattdmJun 17 '11 at 12:45

Billy, fakes as in 3rd party knockoffs like Quantaray. Many cheaper kits created not by the camera manufacturer but resellers include those and they are universally horrible as the lenses are chosen purely for cost rather than quality, in order to get a kit that to the untrained eye looks more impressive at a lower price than the official one.
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jwentingJun 20 '11 at 5:39